1. Field of the Invention
The present invention relates to a visible light communication apparatus for performing communications by using visible light radiated in a space and in particular to the visible light communication apparatus capable of performing communications at high speed by means of a relatively simplified circuit.
2. Description of Related Art
Wireless communications by using radio waves as a communication medium have found many uses in various fields such as cellular phone networks, wireless LANs and short-distance wireless communications.
However, in wireless communications in which radio waves are used as a medium, it is impossible to increase the transmission power, with the influence of electromagnetic waves on the human body taken into account, where the transmitting and receiving are performed near people. Further, since the frequency range of radio waves used in wireless communications has already been allocated in many areas and actually used, it is impossible to use freely frequencies over a wide range. Still further, under special circumstances such as medical institutions, there are some restrictions, for example, the limited use of radio waves.
Thus, in recent years, there has been developed visible light communications in which visible light is used as a communication medium, and a visible light communication system has been proposed in Japanese Published Patent Application No. 2007-266794 A.
In the above-described conventional visible light communication system, visible light is used as illumination light to perform visible light communications, thereby transmitting and receiving information by using white light which is of a daylight color and free of sense of discomfort as illumination light. Therefore, an LED for projecting white light (white light emitting diode) is used as a light projecting element on the side of a transmitter, while a light receiving element for receiving white light to output a light receiving signal is used as a light receiving unit on the side of a receiver.
However, in most cases, an LED which projects white light uses a fluorescent body to generate light, the color of which is complementary to a luminescent color of the LED in order to synthesize the white light thereof. However, due to the lower responsiveness of the fluorescent body, a high frequency signal to be superimposed on the white light is kept low in frequency. Further, the light receiving element for receiving the white light is difficult to operate at high speed on receiving of visible light on which the high frequency signal is superimposed, by which communication speed has been suppressed. For this reason, a conventional visible light communication apparatus has a problem that, for example, where audio signals or image signals great in the amount of digitalized information are superimposed on visible light to transmit or receive them, the communication speed is slow, thus making it difficult to transmit and receive good-quality audio information or good-quality image information.